The use of porous membranes in a variety of filtration applications is well known. For such applications, especially ultrafiltration, membranes with a small pore size and narrow pore size distribution are highly desirable so that precise retention or separation of small molecules with a sharp size or molecular weight cutoff value can be obtained.
Porous membranes are conventionally made by solvent casting or mechanical stretching of polymeric films. For example, film having a microporous structure can be prepared by drawing or stretching at ambient temperatures a crystalline, elastic starting film in an amount of about 10 to 300% of its original length with subsequent stabilization by heat setting of the drawn film under a tension such that the film is not free to shrink or can shrink only to a limited extent. Although the pore size can be varied by manipulating the processing parameters, such as stress, stretching rate and temperature, the size distribution is usually broad and sensitive to any fluctuation of processing conditions.
Another type of popular commercial porous membrane is prepared by solvent casting a solution containing a water-soluble additive which is leached from the cast membrane, thus creating the required porosity. The film casting conditions are manipulated to create a very thin active surface embedded on a microporous spongy mass. Such membranes also have the same problem of having a broad distribution of pore size.
Porous films have been made by casting a solvent solution of a resin or cellulose derivative on a temporary support or substrate as a thin film, after which the solvent is removed or exchanged under carefully controlled conditions. Solvent removal and exchange are very slow and adjustment of processing conditions for close control of product characteristics is difficult. Another process for the preparation of porous membranes also starts from a solution of resin or cellulose derivative and includes casting a film of the solution on a support, after which the membrane is formed by precipitation by immersion of the film in a nonsolvent for the resin or cellulose derivative. In still another method, a polymeric film is extruded under conditions of low melt temperature and high melt stress, the extruded film is annealed while in a relaxed or untensioned state, the film is then uniaxially stretched to develop the desired void structure and the resulting porous film is heat treated in a tension state to stabilize the void structure. Tight control of processing conditions is necessary, since pore size and pore size distribution are highly sensitive to fluctuation in processing conditions.
Although the rated pore size for prior art films may be quite small, many of the pores may actually be substantially larger. While it is desirable for microporous films to exhibit sharp size cutoff, i.e., to sharply delineate the size molecule or particle which will pass from the size which will not, prior art films often do not exhibit this property. If they do, they are very expensive to produce.